1. Field of Invention
The invention generally relates to connectors between devices and carriers and more particularly to connectors that are surrounded by compressible material that prevents delamination of the carrier from the device.
2. Description of the Related Art
Devices such as integrated circuit chips are often connected to carriers that include wiring connections to the integrated circuit chips. The integrated circuit chips can be connected to the carriers using a conductive lead solder. These lead connectors are generally formed as balls on the carrier and/or the chip. The carrier and chip are generally heated to cause the solder to melt, after which the structures are allowed to cool so the solder solidifies. This process is described as a “reflow” process and it bonds the lead solder connection to both the carrier and the chip.
Often, an insulating underfill material is used to fill in the remaining space between the device and the carrier. This underfill helps increase fatigue life of solder interconnections by absorbing some of the stress that results from the difference in the coefficients of thermal expansion of semiconductor devices and ceramic or organic carriers.
Though lead-containing solders have been used for decades and exhibit high yield and reliability due to their extensive utilization, worldwide legislation and environmental concerns have led to considerable interest in the development and use of lead-free solders. One such lead-free solder is SnAgCu, commonly called SAC, which is one of the leading alloys being considered as an alternative to solder connections containing lead. The SAC alloy (available with various levels of Ag and Cu, but typically ranging from 3-4% Ag and 0.5-1% Cu) has numerous advantages including a relatively low melting point, good fatigue life, and compatibility with common lead-free solders. Consequently, SAC is one of the leading candidates for lead-free interconnects between semiconductor devices and chip carriers.
One of the drawbacks in using lead-free solders is that their major constituents tend to experience a relatively large (e.g., 3%) volume expansion upon reflow. Unfortunately, the volume expansion of lead-free solders can force the underfill away from the solder connection, which prevents the underfill from being able to maintain support of the solder when the solder cools back to its original volume. As a result, this large volume expansion upon re-flow prevents some lead-free solders from being used on ceramic or organic carriers that require underfill.